Generated ELF files appear to be subtly incorrect. #22
Description
Per convo w/ @dragonmux:
[ELF] section headers are used specifically and only for linking and that kind of thing - they're there to allow compilers and manipulation tooling easier access to various contained data in the binary
... it's the program headers that control the runtime (loading) environment...
... a lot of tools misuse the headers
In other words, loaders (are) only (supposed to) look at program headers. mspdebug appears to use section headers to flash a binary (subject to other checks that e.g. filter out .bss). By contrast, avrdude uses program headers.
Looking at AT2XT as an example, the first program header appears incorrect:
$ msp430-elf-readelf -lS target/msp430-none-elf/release/at2xt
There are 10 section headers, starting at offset 0x10ac:
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .vector_table PROGBITS 0000ffe0 00085c 000020 00 A 0 0 2
[ 2] .text PROGBITS 0000f800 0000b4 00070e 00 AX 0 0 2
[ 3] .rodata PROGBITS 0000ff10 0007c4 000094 00 A 0 0 4
[ 4] .bss NOBITS 00000200 0000b4 000030 00 WA 0 0 2
[ 5] .data PROGBITS 00000230 000858 000002 00 WA 0 0 2
[ 6] .MSP430.attribute MSP430_ATTRIBUT 00000000 00087c 000017 00 0 0 1
[ 7] .symtab SYMTAB 00000000 000894 000390 10 8 32 4
[ 8] .strtab STRTAB 00000000 000c24 000432 00 0 0 1
[ 9] .shstrtab STRTAB 00000000 001056 000055 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
p (processor specific)
Elf file type is EXEC (Executable file)
Entry point 0xf800
There are 4 program headers, starting at offset 52
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0x0000014c 0x0000014c 0x000b4 0x000e4 RW 0x4
LOAD 0x0000b4 0x0000f800 0x0000f800 0x007a4 0x007a4 R E 0x4
LOAD 0x000858 0x00000230 0x0000ffa4 0x00002 0x00002 RW 0x4
LOAD 0x00085c 0x0000ffe0 0x0000ffe0 0x00020 0x00020 R 0x4
Section to Segment mapping:
Segment Sections...
00 .bss
01 .text .rodata
02 .data
03 .vector_table
A loader/tool that uses the program headers to flash this binary will attempt to "flash" peripheral and/or unmapped memory for 0xb4 bytes, up to address 0x200. At 0x200, flashing/loading stops, and the remaining 0xe4-0xb4 = 0x30 bytes is considered allocated (startup code will fill it in). This is not correct. Program header 00 should be something like:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0x00000200 0x00000200 0x00000 0x00030 RW 0x4
Swapping .bss and .data order in link.x.in, as below, does not fix the problem:
diff --git a/link.x.in b/link.x.in
index a64ec06..0a1ad70 100644
--- a/link.x.in
+++ b/link.x.in
@@ -45,14 +45,6 @@ SECTIONS
. = ALIGN(2);
} > ROM
- .bss : ALIGN(2)
- {
- _sbss = .;
- *(.bss .bss.*);
- . = ALIGN(2);
- _ebss = .;
- } > RAM
-
.data : ALIGN(2)
{
_sidata = LOADADDR(.data);
@@ -62,6 +54,14 @@ SECTIONS
_edata = .;
} > RAM AT > ROM
+ .bss : ALIGN(2)
+ {
+ _sbss = .;
+ *(.bss .bss.*);
+ . = ALIGN(2);
+ _ebss = .;
+ } > RAM
+
/* fake output .got section */
/* Dynamic relocations are unsupported. This section is only used to detect
relocatable code in the input files and raise an error if relocatable code$ msp430-elf-readelf.exe -lS ~/Projects/legacy/AT2XT/target/msp430-none-elf/release/at2xt
There are 10 section headers, starting at offset 0x108c:
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .vector_table PROGBITS 0000ffe0 00083c 000020 00 A 0 0 2
[ 2] .text PROGBITS 0000f800 000094 00070e 00 AX 0 0 2
[ 3] .rodata PROGBITS 0000ff10 0007a4 000094 00 A 0 0 4
[ 4] .data PROGBITS 00000200 000838 000002 00 WA 0 0 2
[ 5] .bss NOBITS 00000202 00083a 000030 00 WA 0 0 2
[ 6] .MSP430.attribute MSP430_ATTRIBUT 00000000 00085c 000017 00 0 0 1
[ 7] .symtab SYMTAB 00000000 000874 000390 10 8 32 4
[ 8] .strtab STRTAB 00000000 000c04 000432 00 0 0 1
[ 9] .shstrtab STRTAB 00000000 001036 000055 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
p (processor specific)
Elf file type is EXEC (Executable file)
Entry point 0xf800
There are 3 program headers, starting at offset 52
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0x0000f76c 0x0000f76c 0x00838 0x00838 R E 0x4
LOAD 0x000838 0x00000200 0x0000ffa4 0x00002 0x00032 RW 0x4
LOAD 0x00083c 0x0000ffe0 0x0000ffe0 0x00020 0x00020 R 0x4
Section to Segment mapping:
Segment Sections...
00 .text .rodata
01 .data .bss
02 .vector_table
Now the loader would be instructed to write .text and .rodata to unallocated memory for the MSP430G2211 part! It's like msp430-elf-ld hardcodes the first program header to load starting at Offset 0 in the ELF file." We need to figure out what causes the bad ELF program header and correct it; while flashing w/ mspdebug is unaffected, other tools may use the incorrect program headers to flash.